1. Field of the Invention
The present invention relates to a focus condition detecting device for a camera which detects the focusing condition of the objective lens through measurment of the light rays coming from an object to be photographed (hereinafter referred to as object light rays) and having passed through the objective lens.
2. Description of the Prior Art
Many focus detecting devices have been proposed wherein object light rays having passed through first and second areas of the objective lens that are symmetric with each other with respect to the optical axis of the objective lens, are re-concentrated or re-converged by a pair of re-imaging lenses to form two images after once concentrated or converged by the objective lens, and the relative positions of the two images are detected to determine the amount and direction of defocus of the object image formed by the objective lens, or the amount and direction of deviation of the focused position of the object image from a predetermined focal plane (whether the object image is in front of or in the rear of the predetermined focal point, i.e. whether a front focus or a rear focus condition is attained). A typical optical system of such focus detecting devices has a construction as shown in FIG. 1. The optical system includes a condenser lens 6 disposed on or in the back of a predetermined focal plane 4 which in turn is to the rear of an objective lens 2. To the rear of the condenser lens 6 are disposed a pair of re-imaging or relay lenses 8 and 10. A pair of photocell arrays 12 and 14 which have CCDs as the light detectors, are disposed on the respective predetermined focal planes of the re-imaging lenses. The images formed by re-imaging lenses 8 and 10 on photocell arrays 12 and 14 are hereinafter referred to as re-images of the object image formed by the objective lens 2. The re-images are nearer or closer to the optical axis 18 and to each other as shown in FIG. 2 when the object image is formed in front of the predetermined focal plane i.e. in the case of front focus. In contrast, the re-images are distant from the optical axis 18 in the case of rear focus. When the objective lens 2 is in in-focus condition, the distance between two corresponding points of the two re-images has a given value determined by the construction of the optical system of the focus detecting device. Accordingly, the focus condition can be basically determined by detecting the distance of the two re-images based on the outputs of the photocell arrays. The outputs of the photocell arrays are correlated through the correlator 16 to provide the defocus signal. The fpllowing method has been known as one of the method for detecting the distance between the two re-images.
With reference to FIG. 3, photocell arrays 12 and 14 are respectively composed of ten and sixteen photodiode cells 21, a1 to a10 and b1 to b16. Assume for the convenience that the reference characters assigned to each cell also represents the output level of the cell. If consecutive ten cells are to be taken from the photocell array 14, seven sets B1, B2 . . . B7 can be made. The focus condition can be detected by determining on which one of the seven sets is formed the re-image that best coincides with the re-image on the photocell array 12. For example, if the re-image on the set B1 of the array 14 best coincides with the re-image on the array 12, that is, if the relationship of a1=b1, a2=b2 . . . a10=b10 is found between outputs of corresponding cells of the two sets a1 to a10 and b1 to b10, the total sum S1 of the absolute values of the differences between outputs of corresponding cells will be the critically as follows: EQU S1=.vertline.a1-b1.vertline.+.vertline.a2-b2.vertline.+ . . . .vertline.a10-b10.vertline. . . . =0 (1)
Thus, the values S1 is smaller than any other values calculated in the same way for the sets other than B1. In other words, the value of S1 is the smallest of the sums of the absolute values of the differences calculated in the same way for all the sets. To find the minimum value, the calculations as given by the formula (1) is made for all the sets and the sums obtained from the calculations are compared with one another. If it is detected the re-image on the set B1 best coincides with the re-image on the set A1, then it is determined whether the set B1 is at a predetermined standard position, nearer or closer to the optical axis than the standard position or more distant than the same, as well as what is the amount of deviation of the set B1 from the standard position. From the result, it is determined whether the objective lens is in the in-focus, front focus or rear focus condition and also the amount and direction of driving required for the objective lens to be brought into the in-focus condition is calculated.
With the above focus condition detecting device, the focus condition is detected based on discrete image signals from the photocell arrays, and hence there is a possibility of a focus detecting error produced dependent on the nature of the re-images. Take an object which is a single point source of light for example. Designated in FIG. 4 at I1, I2 are illuminance distributions of re-images of the light spot which are formed on a line sensor 1 by the condenser lens 6 and re-imaging lenses 8, 10. The line sensor 1 corresponds to photocell arrays 12, 14 in FIG. 1. An interval between adjacent two of graduations on the horizontal axis corresponds to one cell. Since the re-images are images of the point source of light, the range of the illuminance distributions of the re-images fall in one cell of the line sensor, respectively. The focus condition is determined by a distance k between the two re-images I1, I2 on the line sensor. If each of the re-images I1, I2 is moved laterally in one cell, the output from that cell remains unchanged. For example, the line sensor output remains the same at a time when the re-images are positioned at I1, I2 and at a time when they are positioned at inward locations I1', I2' in which the distance k' between the re-images is about two cells shorter than the distance k between the re-images I1, I2. Accordingly, such a distance difference cannot be detected resulting in a focus detection error. The focus detection error is produced not only with an object which is a point light source, but also with an object having a step-like luminance distribution.
In avoiding the above focus detection error, it would be effective to cutting off higher-frequency components from spatial frequency components of the re-images on the line sensor so that the illuminance distribution of the re-images of the light spot would extend over a plurality of cells. However, the focus detecting optical system would be complex in construction since it would require an optical element having the characteristics of a low-pass filter.